Subjects 18 years of age must have experienced at least three migraine headache attacks (with or without aura according to 2004 International Headache Society [IHS] criteria 1.1 and 1.2.1) per month during the 3 months prior to screening and at least four migraine headache days but less than 15 total headache days (migraine or non-migraine) per month during the 3 months prior to screening and must maintain this requirement throughout the last 4 weeks of the baseline period. Approximately 528 subjects from approximately 53 centers in North America will be randomized in a 2:1:2:2:1 ratio to the following treatment groups: placebo, GSK1838262 1200 mg/day, 1800 mg/day, 2400 mg/day and 3000 mg/day. Investigational product will be administered twice daily (morning and evening) with food (e.g., meal or snack).